Upconversion nanoparticles are promising for many applications. For triple-doped nanoparticles (NPs), the luminescence intensity shows a non-linear dependence on the rare-earth ion concentration, making it difficult to obtain bright phosphors with high energy output. We investigated the energy transfer processes in β-NaGdF₄:Yb-Er-Tm NPs and considered strategies for increasing the thulium luminescence intensity, in particular, the use of core–shell structures. The luminescence spectra were analyzed in the short-wavelength infrared (SWIR) and visible (VIS) regions. The Er³⁺ and Tm³⁺ luminescence lifetimes in the VIS region were measured to study the energy transfer processes between the active ions. The quenching of the Tm³⁺ luminescence in the SWIR region was observed. However, both Er³⁺ and Tm³⁺ luminescence bands were observed in the VIS range. We attribute these effects to energy transfer between Tm^3+ 3F₄ → ³H₆ and Er^3+ 4I_13/2 → ⁴I_9/2, which occurs due to overlap of Er³⁺ and Tm³⁺ luminescence bands, and also to competition between Er³⁺ and Tm³⁺ for energy transfer from Yb³⁺. For core–shell NPs, when Tm³⁺ and Er³⁺ are separated into adjacent layers, quenching cannot be avoided, likely due to the mutual diffusion of ions during shell synthesis. The most optimal strategy to obtain luminescence in the SWIR range is to use an inert intermediate shell between the layers containing Tm³⁺ and Er³⁺.

Emission spectral properties and quantum efficiency of upconversion particles NaYF ₄ , SrF ₂ , LaF ₃ , BaF ₂ и CaF ₂ , doped with rare earth ions pair Yb ³⁺ –Er ³⁺ were studied using continuous wave (CW) and pulsed periodic excitation modes in the near infrared (NIR) spectral range. Analysis of the obtained results showed that the intensity ratio of upconversion luminescence in green and red spectral ranges depends on excitation pulse duration. Thus, by changing the pulse duration the spectral properties of upconversion luminescence can be controlled. Crystals with higher phonon energy are more sensitive to the change of pumping mode. Interpretation of results was performed on the rate equation model basis. Using numerical methods for all energy levels involved in the upconversion process the population and depopulation dynamics were obtained with respect to the duration of the excitation pulses. It was shown that about 30 ms was required for the complete population of ⁴ F _9/2 state, from which the luminescence in the red spectral range occurs. When the pulse duration was less than 30 ms, the ⁴ F _9/2 population did not reach a steady state and the intensity of the luminescence in the red part of the spectrum was reduced. The theoretical dependence of the upconversion luminescence intensity in the green and red ranges of the excitation pulse duration for NaYF ₄ :Yb _0.2 –Er _0.02 composition was obtained and demonstrates good agreement with the experimental results.

Upconversion nanoparticles have attracted considerable attention as luminescent markers for bioimaging and sensing due to their capability to convert near-infrared (NIR) excitation into visible or NIR luminescence. However, the wavelength of about 970 nm is commonly used for the upconversion luminescence excitation, where the strong absorption of water is observed, which can lead to laser-induced overheating effects. One of the strategies for avoiding such laser-induced heating involves shifting the excitation into shorter wavelengths region. However, the influence of wavelength change on luminescent images quality has not been investigated yet. In this work, we compare wavelengths of 920, 940 and 970 nm for upconversion luminescence excitation in the thickness of biological tissues in terms of detected signal intensity and obtained image quality (contrast and signal-to-background ratio). Studies on biological tissue phantoms with various scattering and absorbing properties were performed to analyze the influence of optical parameters on the depth and contrast of the images obtained under 920–970 nm excitation. It was shown that at the same power the excitation wavelength shift reduces the detected signal intensity and the resulting image contrast. Visualization of biological tissue samples using shorter excitation wavelengths 920 and 940 nm also reduces signal-to-background ratio (S/B) of the obtained images. The S/B of the obtained images amounted to 2, 6 and 8 for 920, 940 and 970 nm, respectively. It was demonstrated that pulse-periodic excitation mode is preferable for obtaining high quality luminescent images of biological tissues deep layers and minimize overheating. Short pulse durations (duty cycle 20%) don’t result in heating even for 1 W cm⁻² pumping power density and allow obtaining high luminescence intensity, which provides good images quality.

Non-invasive detection of polarized macrophages in tumors is an urgent task in terms of combined antitumor therapy. By analyzing the fluorescence lifetime of the metabolic cofactors—the reduced form of nicotinamide adenine dinucleotide (NADH) and flavins—differences in cellular metabolism of normal tissue, tumor, inflammatory and anti-inflammatory macrophages were demonstrated. In this work we studied changes in the polarization of macrophages obtained from THP-1 monocytes in response to photodynamic therapy with 5-aminolevulinic acid (ALA–PDT). Moderate ALA–PDT in vitro led to changes in M0 macrophages metabolism towards M1 polarization, wherein M1 and M2 macrophages died and were replaced by non-polarized cells. The interstitial distribution of polarized macrophages after ALA–PDT was studied in a mouse tumor model of grafted Lewis lung carcinoma. In response to ALA–PDT, there was an increase in the inflammatory macrophages fraction in the tumor node. Metabolic fluorescence-lifetime imaging microscopy (FLIM) was performed for macrophages in vitro and for tumor cryosections. It was shown that analysis of phasor diagrams for the NADH, flavins, and 5-ALA-induced protoporphyrin IX (PpIX) fluorescence lifetime helps to determine the change in metabolism in response to different modes of PDT at the cellular and tissue levels. These data can be used for post-surgery tissue inspection.

The data about absorption spectra properties of the photosensitized tumor tissues in vivo are very important for the evaluation photosensitizer concentration and influence of the chemical environment on photosensitizer properties in tissue. This information will help one in the appropriate choosing of irradiation light dose and wavelength in the photodynamic therapy (PDT) treatment. The interaction of photosensitizer with tumor tissue may change its absorption spectrum. Moreover the light irradiation during PDT treatment can also affect the photosensitizer chemical structure and hence change its absorption spectrum. The simple technique based on the measurement of reflection spectra by means of fiber optic spectrometer has been developed. This method allows one to evaluate the contribution to the tissue absorption properties due to the presence of photosensitizer in it. The absorption spectra of photosensitized normal and tumor tissues presented in this paper has been obtained during PDT sessions of head and neck tumors. The photosensitizer -- sulphonated aluminum phthalocyanine (Photosence) (NIOPIC, Russia) -- has been injected intravenously in doses 0.5 - 2 mg per kg of body weight. It has been observed that absorption properties of this photosensitizer are not changed significantly in tissue as compared to that of in solution. The absorption spectra widening and red shift (4 nm) has been noticed. The qualitative pharmocokinetic of photosensitizer based on absorption spectra measurements is presented.